Simulation of international space station microgravity directional solidification experiments on columnar-to-equiaxed transition

It is well known that gravity affects solidification of alloys due to the convective effects it induces. As a result, different outcomes are expected if solidification experiments are carried out in near-zero gravity conditions achievable in space. Directional solidification experiments were conducted on board the Material Science Lab (MSL) in the International Space Station (ISS). The experiments, on Al-7 wt.% Si alloys, were carried out with a low gradient furnace (LGF). The LGF is a Bridgman-type furnace insert for the MSL. Numerical simulations for two such microgravity directional solidification experiments are presented and compared with experimental results. A front tracking algorithm to follow the growing columnar dendritic front, and a volume averaging model to simulate equiaxed solidification, were employed simultaneously in a common thermal simulation framework. The thermal boundary conditions for the simulation domain were computed via the temperature readings which were recorded during the experiments. The simulation results include the prediction of columnar-to-equiaxed transition (CET) and average as-cast equiaxed grain diameters, and agreed with the experimental results reasonably. The simulations predict that although an undercooled zone forms ahead of the growing columnar front, thermal conditions in the diffusion-controlled experiments were inadequate to trigger an entirely equiaxed zone without grain refiners.